At 25mm in ambient light (strobes would be cheating!), I can shoot hand-held using IBIS in the area of 1/4s with fair consistency. Without IBIS it's more in the area of 1/10s to 1/15s. So 1.5 to 2 stops for me...

Yep I'd expect 2-3 stops, it really is user dependent of course, relative to how steady you hold the camera. Sounds like the e-m5 might be even better. With the 20mm I wouldn't hestitate to shoot at 1/10th or 1/8th, and might try a few shots at 1/5th or 1/4 if necessary to get possibly get a keeper.